#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
const ll INF = 1e18;
const int MOD = 1000000007;

int main(){
        int n,m;
        cin >> n >> m;
        vector<ll> a(n);
        vector<ll> weight(n);
        vector<ll> cen(n);
        rep(i,n) cin >> a[i];
        ll sum_weight = 0;
        rep(i,m){
                ll x,w;
                cin >> x >> w;
                --x;
                cen[x] ++;
                weight[0] += abs(x);
                sum_weight += w;
        }

        ll cnt = cen[0];
        for(int i=1;i<n;i++){
                weight[i] = weight[i-1] + 2*cnt - m;
                cnt += cen[i];
        }

        bool ok = true;
        ll ans = 0;
        rep(i,n){
                if(sum_weight - a[i] < 0) continue;
                if(weight[i] == 0){
                        if(sum_weight - a[i] <= 0) continue;
                        else{
                                ok = false;
                                break;
                        }
                }
                ans = max(ans,(sum_weight-a[i]+weight[i]-1)/weight[i]);
        }

        if(ok) cout << ans << endl;
        else cout << -1 << endl;
        return 0;
}